The issue of screen protection has been more and more brought to the attention of manufacturers and users with growing popularity of electronic display products such as plasma TVs, liquid crystal TVs, liquid crystal displays, ATM machines, cell phones, PDAs, personal amusement gadgets, information searching machines, media advertising players, etc.
In the past, a layer of protective cover plate was normally placed on the surface that mainly comprised organic plate and soda-lime glass. However, these materials do not have sufficient mechanical properties regarding hardness, mechanical strength, surface scratch resistance and impact resistance. Especially for touch screens, such as the touch screens of PDAs and cell phones, damageable and frequent operations such as writing, scratch and impact on touch screens by fingers, electronic pens, roller pens, and even keys could often cause breakage or surface roughness for screens, which influences the lifetime and the display effect of the devices as a whole.
A touch screen is a display that detects and corresponds to touching in display area, causing a direct interaction with the display without keyboards, mouse devices or touch pads. Touch screens, such as resistive touch screens, capacitive touch screens or projective capacitive touch screens, need a glass substrate to deposit transparent conductive oxides for transmitting signals. In particular, an additional glass cover is normally needed for the purpose of protecting the display.
A resistive touch screen transmits touch signals through a flexible surface cover plate, which normally is a plastic or glass plate coated with a transparent conductive oxide (TCO) film. Touching makes the flexible cover plate deform and is conducted to contact the inner conductive substrate, thereby varying the resistance and current of the circuit. This is generally recognized as a touch event for treatment. A flexible surface layer requires higher scratch resistance and transmissivity especially for outdoor applications. Plastics cannot satisfy the requirements, therefore, a sheet of glass having a thickness of about 0.1 to 0.3 mm is necessary.
Particularly, the projective capacitive touch screen represents the rapid developing trend in the market of touch screens as it has the advantages of high transmissivity, high resolution, and long lifetime; and supports for the multi-touch technology. For this type of capacitive touch screens, glass can be used as a substrate of a coated film or a cover plate. The capacitive touch screen responds to finger's touch through the variation of the capacitance of a circuit, and usually only a glass substrate having a two-side TCO coating is required.
A glass as a substrate is coated with the TCO film. A substrate glass is generally a soda-lime glass coated with TCO (transparent transmission coating). Two sheets of glass coated with TCO film layers forms a capacitor through a thin spaced room. What is important is that the top layer glass protects the display from being damaged, and thus high surface scratch resistance and strength are desired.
In addition, other types of touch screens such as surface acoustic wave touch screens, optical touch screens all need glass cover panels or substrates of high transmissivity and high strength.
The substrate and cover plate in the application field of touch screens require that a glass should be subjected to strengthening and/or toughening for further increasing the strength and toughness of the glass. Generally, a glass thickness is supposed to be in the range of 0.3 mm to 1.5 mm when application requires strengthening since the physical tempering is applicable only to a glass having a thickness of greater than 3 mm. Therefore, chemical treatment is necessary for strengthening a glass having a thickness ranging from 0.3 mm to 1.5 mm.
The glass surface strengthening technology includes not only the general air tempering (air jet strengthening) process, but also the chemical tempering technology. The nature of the chemical tempering technology is to change the structure of the glass surface, thus increasing the strength of the glass surface. The chemical tempering can also be named chemical strengthening.
The chemical strengthening is generally divided into 1) alkylating treatment of the glass surface; 2) plating a layer of glassy material having a low expansion coefficient on the glass surface; 3) alkali metal ion exchange. The chemical strengthening mentioned here refers to ion-exchange strengthening. Generally speaking, ion-exchange strengthening is further executed through two processes: high temperature ion exchange and low temperature ion exchange. And the ion-exchange strengthening is also named ion-exchange toughening.
The high temperature ion-exchange strengthening refers to forming an altered layer on the glass surface with a glassy material having a low expansion coefficient above the strain temperature of a glass. A typical high temperature ion-exchange is to heat a glass comprising Na2O and K2O to a temperature above the strain temperature, impregnating the glass into molten salt comprising Li+ at a temperature lower than the softening temperature, and heating the glass and the molten salt to a temperature above the strain temperature; the glass has a loose network at this time, and the surface can be altered easily, thereby promoting the ion-exchange between Na+ or K+ and Li+ occurring between the glass and the molten salt. After a period of ion-exchange between Li+ and Na+ or K+ on the glass surface, the glass is taken out for annealing and cooling to room temperature, and then a layer rich in Li+ ions is formed on the glass surface. Since a Li+ layer has an expansion coefficient far lower than that of an ion layer comprising Na+ or K+ (a glass rich in sodium and potassium, and a glass rich in lithium have different expansion coefficients), the contractions on the exterior and in the interior of a glass are different during cooling. Therefore, a residual compressive stress layer is formed on the glass surface, and a tensile stress layer is generated inside the glass. If the glass comprises Al2O3 and TiO2, a TiO2—Al2O3-4SiO4 crystal having an even smaller expansion coefficient is formed when ion-exchange taking place, and an extremely large compressive stress will be generated after cooling. However, the high temperature ion-exchange may cause optical distortion easily, and the glass can be deformed easily. And the molten salt has a high temperature during production, generates a large amount of volatiles and pollutes environment. Further, the molten salt is apt to be failure easily.
The low temperature ion-exchange strengthening is widely used, of which the mechanism comprises impregnating the glass into a molten salt of an alkali metal compound wherein the ion has a radius larger than that of the alkali metal ion contained in the glass under a temperature lower than the strain temperature of the glass. Ions having large volumes in the molten salt squeeze into spaces occupied originally by ions of small volumes in the glass network, and ions of small volumes are exchanged into the molten salt. When the glass is cooled, the glass network shrinks. As ions of large volumes need larger spaces, a compressive stress is formed on the glass surface. The surface compressive stress remains inside the glass when cooled, and thus, a compacted compressed layer is formed on the glass surface. Presence of such a compressed layer can reduce fine cracks on the glass surface with a prestressed layer forming on the glass surface, whereby the bending resistance and impact resistance of the glass are tremendously increased.
Smaller ions in a glass are exchanged by larger ions in a chemical salt, for example, smaller Na+ ions in a glass is exchanged by larger K+ ion by the commonly known KNO3 salt bath.
For the purpose of promoting an efficient ion-exchange process, at least one smaller alkali ion, in particular Li+ or Na+, is an indispensable component in a glass that allows for being exchanged by larger ions in a salt such as KNO3. Under the conditions of not losing other characters, the amount of the alkali ion should be as high as possible for providing sufficient exchange sites. In addition, Al2O3 is also an indispensable component for providing a high strength glass material, and what is important, larger networks can be formed as diffusion channels for housing alkali metal ions, which in turn will result in a rapid and effective ion exchange.
The ion exchange process generates a compressive stress of several hundreds MPs on the glass surface, and a tensile stress in the center of the glass correspondingly due to the fact that the surface compressive stress inhibits small defects from spreading and thus an strengthening effect is obtained when an outside load is applied.
Generally, a depth of ion exchange layer (which is called DoL) of several tens of micrometers can be obtained on the glass surface by ion exchange process, which is of highly scratch resistance against outside forces.
In order to satisfy the requirement of application as a cover plate of a touch screen, a rapid and effective strengthening treatment is very important in terms of practical perspective. CN 101337770 A records that treatment is conducted for 3 to 8 hours in a temperature ranging from 430 to 490° C. And CN 101508524 A records that the glass needs to be treated for 8 hours at 420° C. or 5 hours at 500° C. for obtaining a DoL of above 40 micrometers. However, a high salt bath treatment temperature or a long treatment time tends to loose the network of the glass surface rapidly, producing a lower surface compressive stress.
In addition, a “green” glass without harmful substances has become the new trend of electronic products. Formulations of a glass disclosed in US 2008/0286548, U.S. Pat. No. 5,895,768, CN 101575167A, CN 101337770A and CN 101508524A are all involved with As2O3, Sb2O3, Cl2 or F2 as an agent for glass refining, which, however, imposes limitations on the possibilities of their wide use in electronic industry.
Moreover, for a touch screen, especially for a touch screen glass used in outdoor public places, there is a possibility of bacterium spreading by fingers, which requires an antibacterial function for a new generation touch screen. According to the report of US 2007/0172661, the above function on the surface of a normal flat glass or a glass ceramics is achieved mainly by a film coating process or an ion-exchange process. Its principle is to introduce silver ions on the glass surface for absorbing and killing bacteria.